1. Field of the Invention
This invention relates generally to a fastener used for installing a waterproof membrane on a roof. More particularly, it relates to a stress reliever assembly having top and bottom portions through which at least one screw is inserted to secure the membrane to the roof.
2. Description of the Prior Art
A variety of screw and stress reliever type fastener systems have been designed for securing roof covering materials to roof decks made of steel, gypsum, tectum, or wood. Such roof covering materials typically include a water impervious membrane, and frequently include successive layers of materials; insulation is frequently placed between the deck and the membrane. Such systems generally include an elongated screw which penetrates a flat plate (washer). The plate (sometimes referred to as a stress plate) urges the roof covering membrane and insulation downwardly when the screw is tightened to a rooftop sublayer and tends to prevent the membrane from pulling vertically or horizontally over the head of the screw.
Screw and plate fasteners are most frequently used on commercial buildings having a flat roof. In general, a layer of insulation is frequently placed over a deck. Membrane, typically marketed in rolls, is then laid out over the insulation. The membrane edges along the perimeter of the roof are usually fixed by battons or other conventional techniques. Fasteners are then installed along the interior edge of the membrane sheet, the distance from the edge and the distance between fasteners being determined in accordance with the types of decking and insulation material used and the anticipated conditions. More specifically, a minimum force which will cause the membrane to tear away from the fastener (or the fastener to pull out of the roof) is prescribed by the architect or designer. The number of fasteners per unit linear distance is chosen to ensure that the membrane will be retained.
To install a typical fastener, a pilot hole may be employed. If so, it is first drilled at the desired location through the roof membrane, insulation, and deck. A plastic or metal screw is inserted through a retaining stress plate and then driven into the hole, engaging the roof deck so that the plate is held tightly against the membrane. After the fasteners have been installed along the edge of the first sheet, a successive sheet of membrane is arranged with one edge overlapping the edge of the first sheet of membrane, thereby covering the fasteners. That overlapping edge of the successive sheet is bonded (e.g., chemically or by heating) to the sheet secured by the fasteners. The other edge of the succeeding sheet is fixed to the insulation and deck by fasteners in the manner previously described. Thus, the roof is covered by overlapping sheets of membrane.
Metal screws with plastic plates proposed for use as roofing anchors are described in, for example, DeCaro U.S. Pat. No. 4,361,997 issued Dec. 7, 1982; Hartman U.S. Pat. No. 4,780,039, issued Oct. 25, 1988; Dewey U.S. Pat. Nos. 4,380,413 and 4,545,270, issued Apr. 19, 1983 and Oct. 8, 1985, respectively; and Hasan U.S. Pat. No. 4,663,910, issued May 12, 1987, U.S. Pat. No. 4,712,959, issued Dec. 15, 1987, and U.S. Pat. No. 4,757,661, issued July 19, 1988.
Problems have been encountered when conventional screw and plate fasteners are employed in such roof applications. Wind blowing over the membrane tends to create a negative air pressure, which in turn tends to cause the membrane to pull laterally from the fastener. To assist in preventing the membrane from tearing out from the fastener due to such lateral forces, downwardly directed cleats, lugs, spikes, ribs or other protrusions on the underside of the stress plate have been provided. These engage the membrane as the screw is tightened into the rooftop. See, for example, Murphy U.S. Pat. No. 4,787,188, issued Nov. 29, 1988; Reinwall, et al U.S. Pat. No. 4,726,164 issued Feb. 23, 1989; and Francovitch U.S. Pat. No. 4,476,660, issued Oct. 16, 1984. However, such plates are typically not suitable for certain types of roofs, particularly those roofs sometimes used in warm climates, where the membrane is applied directly over a deck of relatively hard material, without intervening insulation. Also, in some applications where particularly tough membranes are employed or low density insulation is used, the protrusions sometimes fail to adequately engage the membrane. Further, over time the screw tends to cease to provide the original level of preload tension relative to the membrane. This may happen because the insulation deteriorates and shrinks due to, e.g., harsh weather conditions, or because vibrations cause the screw to back out from the deck material. Such a loss of tension can also cause the lugs on the underside of the stress plate to lose engagement with the membrane, making the membrane more susceptible to pull out due to lateral forces. These conditions can also result in the head of the screw popping out, i.e., protrude from the surrounding roofing material, which in turn leads to damage to the overlying membrane.
A number of mechanical systems have been proposed for preventing separation of the screw from the plate in roofing fasteners. Back out can be prevented by preventing the screw from turning (in a reverse direction) relative to the roof. This could be accomplished with a broad headed screw having lugs, spikes, ribs or the like on the underside of the head which would engage the membrane and prevent counter-rotation. However, rotation of such devices, during installation would tend to tear or otherwise damage the membrane. Accordingly, fastener systems have been proposed which include a plate with anti-rotation structure (such as spikes) to engage the membrane, a screw, and a mechanism to prevent counter-rotation of the screw relative to the plate. Such a system is described in the aforementioned Dewey U.S. Pat. No. 4,380,413. Projecting pawls on the head of the screw, cooperate with projections on a plate, much like a ratchet system, to prevent counter-rotation after installation.
A similar system employing a ratchet mechanism to prevent a screw from backing out is described in Giannuzzi U.S. Pat. No. 4,763,456, issued Aug. 16, 1988. However, these various ratchet structures tend to give the fastener assembly an undesirably high profile, and may be susceptible to failure due to loss of tension or breakage of the ratchet members if overtightening occurs. Other systems employ a threaded connection between the plate and fastener. For example, the aforementioned DeCaro U.S. Pat. No. 4,361,997 describes a fastener with upper and lower sets of threads with an intervening unthreaded area which cooperates with a stress plate having anti-rotation structures on its underside. The lower set of threads are coupled to the plate prior to installation. The upper threads engage the plate after the screw is substantially driven into the roofing deck. The anti-rotation structures engage the roof membrane and prevent the plate from turning.
The prior art stress reliever plates are disadvantageous in a number of respects. Perhaps most importantly, the interaction of the teeth, spikes or lugs on the underside of the stress reliever plate may cause wrinkles in the membrane during tightening of the anchor. In addition, the profile of the stress reliever plate and anchor must be relatively low and without sharp corners so that the membrane will not be torn if the anchor is stepped on. The spikes must be sufficiently thick to withstand lateral forces exerted by wind lift. However, the bigger the spike, the more wrinkles created in the membrane, and the spike must be sufficiently sharp (pointed) to penetrate the membrane. Metal stress relievers provide strength, but tend to be corrosive, and in some instances, are subject to galvanic effects.
Other systems have been proposed which employ a cap over the head of the fastener (see, e.g., Verble U.S. Pat. No. 4,658,558, issued Apr. 12, 1987; Francovitch U.S. Pat. No. 4,520,606, issued June 4, 1985; Beneze U.S. Pat. No. 4,620,402, issued Nov. 4, 1986) or resilient spring mechanisms to maintain tension (see, e.g., Hewison U.S. Pat. No. 4,616,455, issued Oct. 14, 1986). Application of a bonding or sealing agent over the head of a fastener, between a stress reliever plate and the membrane, or both have also been proposed. See Sandquist U.S. Pat. No. 4,074,501, issued Feb. 21, 1978, and Francovitch U.S. Pat. Nos. 4,455,804 and 4,467,581, issued June 26, 1984 and Aug. 28, 1984, respectively. Still other systems rely on a nut or similar element disposed on the lower end of the screw beneath the rooftop to hold the fastener in place. See Sargent U.S. Pat. No. 4,727,699, issued Mar. 1, 1988. These fasteners are only partly effective in preventing the fastener from backing out and require additional structure for that purpose.
Roofing fastener systems with provisions for preventing the head of the screw from protruding beyond the top of the plate, e.g., in the event of loss of installation tension, have also been proposed. For example, a system where the washer includes a flexible ring about the aperture that receives the screw is described in Dewey U.S. Pat. No. 4,380,413. Another such system employing a plastic washer having a resilient rib which engages the screw head to hold it in place is described in the aforementioned Hasan Pat. Nos. 4,712,959 and 4,757,661.
Another class of membrane fasteners includes multiple components for engaging the membrane, in addition to the fastener used to secure at least one of the components to the roof. Examples of such fasteners are described in Francovitch U.S. Pat. No. 4,520,606, issued June 4, 1985 (a lower plate is secured to the The intermediate membrane layer by linear fasteners passing into the lower plate or by head and socket engagement); Hickman U.S. Pat. No. 4,586,301, issued May 6, 1986 (a core with outreaching arms is secured to the roof, is covered by the membrane, and secured in place by a clip which fits over the core unit cooperating with a locking clamp); Tomaszewski U.S. Pat. No. 4,617,771 issued Oct. 21, 1986, Boginski U.S. Pat. No. 4,624,092 issued Nov. 25, 1986, Backenstow, et al U.S. Pat. No. 4,649,686 issued Mar. 17, 1987, and Marston U.S. Pat. No. 4,651,490 issued Mar. 29, 1987 (snap-like fasteners wherein a lower member having an annular boss is anchored to the roof, the membrane is draped over the lower member, secured in place by a snap ring secured to the boss, and a locking cap is added); Tripp U.S. Pat. No. 4,744,187 issued May 17, 1988, (upper and lower plates with a series of annular concentric ridges); and Gasser U.S. Pat. No. 4,757,662 issued July 19, 1988 (a two-part locking ring-type fastener). The major problem with these multi-component fasteners is that they have unacceptably high profiles. Furthermore, numerous parts must be employed during the membrane installation process.
In fields totally unrelated to the field of the present invention, devices have been described which include generally parallel plates with a hinge or coupling system along one edge and a series of pins extending upwardly from one plate toward the other. See the clothing fabric fastening device of Trundy U.S. Pat. No. 3,149,386 issued Sept. 22, 1964. In another device (Burns U.S. Pat. No. 4,378,617, issued Apr. 5, 1983) used to suspend agricultural shade cloth, shaft-like pins extend into holes in the top plate. Apertures are provided in the illustrated embodiment for lashing ropes and the like.
Despite the large number of fasteners available, a need remains for a roof fastening system that provides a high pull out rating, simplicity in structure and ease of installation. A fastener which would avoid wrinkling of the membrane during installation and which would apply equalized pressure to the membrane under a stress plate would also be highly desirable.